One measured quantity correlated with extinction is a concentration, for example. Optical sensors for determining a concentration of a substance in a medium often operate with wavelengths of ultraviolet (UV), visible (VIS), near-infrared (NIR), and/or middle-infrared (MIR) light. An extinction measurement is thereby usually implemented. In the following, the problem shall be described using the measured quantity of the concentration, but analogously occurs for additional measured quantities correlated with extinction.
The measurement principle for determining concentration is based upon the Lambert-Beer law. A linear dependency between the extinction of light and the concentration of a substance to be measured exists for low and medium concentrations, with the formula:
      E    =                  log        ⁡                  (                                    I              o                        I                    )                    =              ɛ        ·        c        ·        OPL              ,where E is the extinction, I0 is the intensity of the incident light, I is the intensity of the transmitted light, ε is the extinction coefficient, c is the concentration, and OPL is the optical path length, viz., the path by which the radiated light penetrates through the medium to be measured. In this regard, see also FIG. 1. The intensity I0 may be determined via a reference measurement. At higher concentrations, this linear dependency no longer applies.
A light source 1 sends radiation through optical window 2 into the medium 5. The radiation remaining after passage through the medium 5 is measured at the detector 3, often after passing through a filter 4. The light is thereby converted by a photodiode into photoelectric current. The final conversion of the determined extinction into the concentration takes place by means of a mathematical model, e.g., an association table, in an associated measuring transducer (not depicted).
The extinction has several different causes. On the one hand, there is the scattering at particles located in the medium, and, on the other hand, there is the absorption of the radiation in the medium and at the particles. As additional causes, diffraction and reflection, by this measurement principle, play a subordinate and negligible role.
If the extinction is measured as described above, the proportions of absorption and scattering cannot be separated, and a generally valid determination of the concentration of a substance to be determined is not possible. A separate model is required for each medium. The medium to be measured (sewage sludge, for example) is a member of class of similar media (i.e., various sludges) that, however, vary in their composition and therefore in their ratio of absorption to scattering. If only the extinction is measured, this ratio is plugged as a constant into the model, which is why it is also valid only for a specific substance. Problems may thereby occur. If the composition of the medium changes from system to system, a different model is required for each system. After initialization, the model must be adapted. This is, for one thing, complicated, and not always possible with continuously running systems. If the process medium changes over time (for example, summer to winter), the model must be adapted in turn, or a switch between different models must take place.